This invention is directed to crystal filters and to crystal filter fabrication techniques.
A basic crystal filter network is shown in U.S. Pat. No. 2,988,714, incorporated herein by reference. To construct such networks, two or more crystals are arranged in a "ladder" network including coupling capacitors coupled to circuit ground so as to form a tuned circuit "transformer" network. Although the basic design of crystal filters has not drastically changed from this crystal "transformer" filter network, operation of such filters at high frequencies and with narrow passbands presents design problems not easily overcome. Most of the developments in the crystal filter art have been directed to the use of different coupling elements between the crystals for tuning them to desired resonance and so as to achieve optimum filter characteristics.
The state of the art in crystal filter fabrication has, up to the present, utilized discrete capacitors, such as "chip" capacitors associated with thick film circuits where necessary for coupling. Usually, the crystal filter is constructed on a printed circuit board with hardwire interconnections among the elements.
Using conventional techniques, it is difficult to mass produce crystal filters while meeting highly precise filter characteristics design criteria. In order to build crystal filters meeting the specifications necessary for current radio applications, it is necessary to utilize very small value coupling capacitors. The capacitance values of these capacitors must be controlled within a narrow tolerance. In order to achieve optimum design using conventional techniques, the fabrication process becomes extremely costly.